1. Field of the Invention
The invention concerns the control of stepper motors, especially stepper motors designed to operate in severe environments including cryogenic environments, environments subject to high levels of interference, vacuum (space), and so on. To be more precise, the invention is not so much concerned with the service operation of such motors as with the preceding development and testing of stepper motors and their control devices which, in practice, are periodic signals.
2. Description of the Prior Art
In practice, to test a stepper motor under its future service operating conditions, the usual procedure is to test all of the mechanisms in which the stepper motor is to be integrated. The combination of the resulting overall dimensions of the device under the test and the constraints associated with simulating the severe service operation environment means that in practice the motor is inaccessible for installing measurement sensors to monitor the operation of the motor and the torque that it produces.
It, therefore, appears essential, if stepper motors are to be tested accurately, to test them in isolation, independently of the mechanisms into which they are designed to be integrated; however, tests of this kind presuppose the possibility of simulating as accurately as possible the actual conditions under which power is supplied to the stepper motors in service, assuming this is known. Stepper motor control simulation systems currently available on the industrial market can simulate only a small number of typical, conventional power supply regimes which are in practice far removed from real power supply regimes, especially in a severe environment. Various known electronic circuit boards and systems are adapted to generate conventional pulse, squarewave, sine/cosine control signals. A solution offering slightly better performance is put forward in the document FR-2 440 642. In the case of a motor with four windings, this document teaches the digital application to the windings, to one winding or to two windings simultaneously, of currents between a zero level and a maximum level, for example at levels representing one third and two thirds of maximum level. However, given the necessarily limited number of such intermediate levels, the disclosure of this document enables only highly approximate simulation of a real curve composed of a multitude of intermediate levels between its extreme levels.
The need to be able to accurately simulate the power supply regime and the operation of a stepper motor is particularly crucial in the case of space applications. Stepper motors are being used more and more frequently on satellites where they are called upon to operate in very severe environmental conditions, in particular in a vacuum. In some cases stepper motors are even used in a cryogenic environment, especially as drive motors in infrared observation systems. Cryogenics is the technology of low temperatures, meaning temperatures below -173.degree. C. approximately, at which the major gases (hydrogen, nitrogen, oxygen and air) are in the liquid state. The range of temperatures approaching absolute zero (0 K.) that is to say temperatures of around 0 K. to 4 K., are particularly important.
It is obvious that simulating at ambient temperature the behavior of a stepper motor at cryogenic temperatures involves eliminating all temperature effects. It is, therefore, necessary to simulate the power supply regime of a stepper motor for a cryogenic application in terms of current rather than in terms of voltage. The previously mentioned document FR-2 440 642 provides a partial response to this requirement.
The somewhat rudimentary nature of the devices currently known for generating stepper motor control signals, in other words the devices determining the stepper motor's power supply regime, explains the empirical approach adopted until now in developing mechanisms designed to integrate a stepper motor. Based on a relatively crude evaluation of the torque required, a choice is made from the conventional type signals (pulse, squarewave, sine/cosine, etc.) of the signal type which seems most appropriate to obtaining the required torque. The power rating of the stepper motor to provide the required torque given the selected simplified power supply regime is then determined, including a margin to allow for the imprecise knowledge of how the stepper motor will actually behave in subsequent service.